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United States Patent |
5,593,511
|
Foster
,   et al.
|
January 14, 1997
|
Method of nitridization of titanium thin films
Abstract
Titanium films are nitrided at temperatures less than 650.degree. C., and
preferably between 400.degree. C. and 500.degree. C., by treating the
titanium film with a plasma formed from a nitriding gas at elevated
temperatures. The plasma is created by subjecting the nitriding gas to RF
energy, preferably an electrode having a frequency of 13.56 MHz or less.
The reaction temperature can be reduced by lowering the plasma frequency
to less than 500 KHz. This provides for nitridization at temperatures of
480.degree. C. and lower.
Inventors:
|
Foster; Robert F. (Phoenix, AZ);
Hillman; Joseph T. (Scottsdale, AZ)
|
Assignee:
|
Sony Corporation (Tokyo, JP);
Materials Research Corporation (Park Ridge, NJ)
|
Appl. No.:
|
567830 |
Filed:
|
December 6, 1995 |
Current U.S. Class: |
148/238; 148/237; 257/915; 257/E21.3; 257/E21.584; 438/656 |
Intern'l Class: |
C23C 008/20 |
Field of Search: |
148/238,237,669
427/38,39,228
437/238
204/192.1
|
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| |
Other References
Wolf, Stanley, Ph.D. and Richard N. Tauber, Ph.D., Silicon Processing for
the VLSI Era, vol. 1, Process Technology, Dec. 1986, Lattice Press, Senset
Beach, CA, pp. 574-577.
Dec. 1987 Proceedings Fourth International IEEE VLSI Multilevel
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Santa Clara, CA.
|
Primary Examiner: Kastler; Scott
Attorney, Agent or Firm: Wood, Herron & Evans, P.L.L.
Parent Case Text
This application is a continuation of application Ser. No. 08/253,394,
filed Jun. 3, 1994, now abandoned.
Claims
We claim:
1. A method of applying a titanium nitride layer over a titanium film
comprising:
exposing said titanium film to a plasma selected from the group consisting
of a nitrogen plasma and an ammonia plasma at a temperature less than
about 500.degree. C. for 20 to 300 seconds, wherein said plasma is created
by subjecting a nitriding gas selected from the group consisting of
ammonia and nitrogen to an Rf electrode, thereby forming a titanium
nitride layer on said titanium film;
and depositing on said titanium nitride layer a titanium nitride film by
chemical vapor deposition wherein said titanium nitride film is formed
from titanium tetrachloride and a gas selected from the group consisting
of nitrogen, ammonia and mixtures of nitrogen and ammonia.
2. The method claimed in claim 1 wherein said nitriding gas is ammonia.
3. The method claimed in claim 1 wherein said RF electrode has a frequency
of 400 KHz to about 33 MHz.
4. The method claimed in claim 2 wherein said temperature is less than
about 500.degree. C. and said frequency is less than about 500 KHz.
5. The method claimed in claim 1 wherein said nitriding gas is passed in
contact with said RF electrode at a flow rate of from about 10 sccm to
about 10,000 sccm.
6. The method claimed in claim 5 wherein said plasma frequency is less than
about 500 KHz and said temperature is less than about 500.degree. C.
7. The method claimed in claim 1 wherein said plasma is created by passing
said gas through a showerhead nozzle above said titanium film and wherein
said plasma is generated at said showerhead.
8. The method claimed in claim 7 wherein said showerhead is an RF electrode
and said plasma is generated by said showerhead as nitridization gas
passes through said showerhead.
9. The method claimed in claim 7 wherein said nitridization gas is
introduced into a flow chamber and develops a flow profile within said
chamber, wherein said RF electrode comprises a showerhead and said plasma
is created as said gas passes through said showerhead.
10. The method claimed in claim 1 wherein said RF electrode is positioned
from about 10 cm to about 10 mm from said titanium film.
Description
BACKGROUND OF THE INVENTION
In the formation of integrated circuits (IC's), thin films containing metal
elements are often deposited upon the surface of a substrate, such as a
semiconductor wafer. Thin films are deposited to provide conducting and
ohmic contacts in the circuits and between the various devices of an IC.
For example, a desired thin film might be applied to the exposed surface
of a contact or via on a semiconductor wafer, with the film passing
through the insulative layers on the wafer to provide plugs of conductive
material for the purpose of making interconnections across the insulating
layers.
Particularly, certain aspects of IC components are degraded by exposure to
the high temperatures normally related to traditional thermal chemical
vapor deposition (CVD) processes. At the device level of an IC, there are
shallow diffusions of semiconductor dopants which form the junctions of
the electrical devices within the IC. The dopants are often initially
diffused using heat during a diffusion step, and therefore, the dopants
will continue to diffuse when the IC is subjected to a high temperature
during CVD. Such further diffusion is undesirable because it causes the
junction of the device to shift, and thus alters the resulting electrical
characteristics of the IC. Therefore, for certain IC devices, exposing the
substrate to processing temperatures of above 800.degree. C. must be
avoided, and the upper temperature limit may be as low as 650.degree. C.
for other more temperature sensitive devices.
Furthermore, such temperature limitations may become even more severe if
thermal CVD is performed after metal interconnection or wiring has been
applied to the IC. For example, many IC's utilize aluminum as an
interconnection metal. However, various undesirable voids and extrusions
occur in aluminum when it is subjected to high processing temperatures.
Therefore, once interconnecting aluminum has been deposited onto an IC,
the maximum temperature to which it can be exposed is approximately
500.degree. C., and the preferred upper temperature limit is 400.degree.
C. Therefore, as may be appreciated, it is desirable during CVD processes
to maintain low deposition temperatures whenever possible.
In one particular application, a thin film of titanium must be deposited
over silicon contacts prior to depositing the metal interconnection in
order to provide low contact resistance. In state-of-the-art integrated
circuits, this titanium deposition is followed by the deposition of a
titanium nitride (TIN) diffusion barrier layer. The diffusion barrier
prevents aluminum interconnection from diffusing into the silicon contact
and causing a short circuit. The diffusion barrier is also needed in the
case of tungsten metalization. In this case, the TiN diffusion barrier
prevents the tungsten precursor, tungsten hexafluoride, from diffusing
into the silicon contact and reacting with the silicon.
There are low temperature physical techniques available for depositing
titanium nitride (TIN) on temperature sensitive substrates. Sputtering is
one such technique involving the use of a target of layer material and an
ionized plasma. To sputter deposit a film, the target is electrically
biased and ions from the plasma are attracted to the target to bombard the
target and dislodge target material particles. The particles then deposit
themselves cumulatively as a film upon the substrate. Titanium Nitride may
be sputtered, for example, over a silicon substrate after various contacts
or via openings are cut into a level of the substrate.
The conformality of sputtered TiN is poor in the high aspect ratio contacts
which are used in state-of-the-art integrated circuits. This poor
conformality leads to poor diffusion barrier properties. Therefore, TiN
films deposited by chemical vapor deposition (CVD) are preferred over
sputtered films. TiN films deposited by CVD from TiCl.sub.4 and NH.sub.3
provide 100% conformality over very high aspect ratio contacts.
The HCl which is a byproduct of this TiN deposition reaction can etch the
titanium contact layer. In order to avoid etching of the titanium, the
upper layer of the titanium must be nitrided prior to TiN CVD. This
nitridization can be done by rapid thermal annealing at 800.degree. C. in
a nitrogen or ammonia ambient. This temperature is far too high for the
shallow junctions which are present in most state-of-the-art integrated
circuits. Therefore, a low temperature nitridation process is necessary.
Annealing the titanium film in ammonia at 650.degree. C. results in a very
poor quality nitridation of the titanium surface as evidenced by the lack
of a characteristic gold color. When TiN is deposited by chemical vapor
deposition over the thermally annealed titanium, the result is the same as
with the unannealed titanium. Prior attempts at annealing using an ammonia
plasma have been unsuccessful.
SUMMARY OF THE INVENTION
The invention provides a process for nitriding thin films of titanium at
temperatures ranging from 400.degree. C. to 650.degree. C. More
particularly, the present invention provides a method to nitride a
titanium film using an ammonia anneal at less than 650.degree. C. Using an
RF ammonia plasma results in an improved outcome, permitting the lower
temperatures. Further, generating the plasma near the titanium surface
significantly improves the nitridization.
When a titanium film is annealed in a 13.56 KHz ammonia plasma at
650.degree. C. the surface of the titanium turns gold in color indicating
that the surface has been converted to TiN. When TiN is deposited by
chemical vapor deposition over the plasma annealed film, the result is a
smooth and conformal film with no evidence of pitting, cracking or
delamination. This is apparently because the thin titanium nitride layer
formed on the surface of the titanium is protecting the titanium from HCl
attack.
A preferred embodiment of the present invention utilizes a showerhead
affixed to the end of a cylinder which is biased with RF energy to create
a plasma of ammonia or nitrogen passing through the nozzle. The showerhead
RF electrode is located close to a rotating susceptor which rotates a
titanium-coated substrate.
The gas to be activated is introduced into the cylinder and travels through
the showerhead and through the RF field of the showerhead where it is
excited into a plasma as it is dispersed through the openings of the
showerhead uniformly over the surface of the substrate. The rotating
susceptor rotates the substrate and pumps the plasma along the substrate
in a laminar flow pattern to ensure maximum radical concentration at the
surface of the titanium film. The cylinder improves the flow pattern of
the gas to enhance the laminar flow delivery.
Reducing the frequency of the RF plasma further improves the affect of the
anneal, When the film is annealed in an ammonia plasma created by a 450
KHz electrode the temperature can be significantly reduced, i.e., to less
than 500.degree. C. The gold color achieved with this anneal is deeper
than the gold color achieved at 650.degree. C. and 13.56 MHz. This
indicates a higher quality TiN surface layer has been formed, and suggests
that the anneal temperature can be further reduced when a low frequency RF
plasma is used.
The invention and the particular advantages and features of the present
invention will now be described in detail below with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE is a side view, partially in cross-section of a deposition
chamber used to practice the methods of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, a titanium film is nitrided by treating
it with a plasma created from a nitriding gas such as nitrogen or,
preferably, ammonia. According to the present invention, the titanium film
may be formed over a variety of different semiconductor substrates. These
would include silicon, thermal oxides and ceramics. The titanium film can
also be deposited over patterned wafers including those having contacts
with silicon and aluminum. The particular surface upon which the titanium
is deposited and the method of depositing the titanium film do not form
part of the present invention.
Two nitriding gases can be used in the present invention. These are ammonia
and nitrogen. Ammonia is preferred because of its better reactivity. The
plasma is created by simply subjecting the nitriding gas to an RF
electrode at elevated temperature and reduced pressure. The titanium film
is then contacted with this plasma, thereby forming titanium nitride.
Preferably for use in the present invention, the RF electrode will have a
minimum power of 100 watts. The maximum power is the power at which
devices are damaged, i.e., about 5 Kilowatts. Approximately 250 watts is
adequate, The frequency of the RF electrode should be from about 55 MHz to
about 33 KHz, As the frequency is lowered, the temperature of the
treatment can also be reduced, The upper frequency is a function of
Federal Communication Commission regulation and equipment availability.
However, as described below, lower frequencies are generally preferred.
In order to preserve the underlying titanium film and substrate, the
temperature should be kept at less than 650.degree. C. As the frequency of
the electrode is reduced, the temperature can also be reduced. Generally,
the reaction temperature can vary from about 650.degree. C. down to about
300.degree. C. Preferably, the temperature will be less than 500.degree.
C., preferably from about 400.degree. to about 450.degree. C., which will
provide for excellent nitridization and reduce thermal degradation of the
underlying substrate and titanium film. By lowering the RF frequency, the
temperature can also be lowered. As the frequency is reduced to less than
500 KHz, the temperature can be reduced below 500.degree. C. At 450 KHz,
the temperature can be 450.degree. C.
The time, pressure and flow rates, as well as temperature, can all be
varied to increase or decrease the reaction rate of the nitridization.
Generally, the minimum flow rate of the nitridization gas should not be
less than about 10 sccm. At flow rates above 5,000 sccm there is increased
unvented gas without any benefit although flow rates above 10,000 sccm
will function. But precise flow rate is not critical for practicing the
present invention. Therefore, about 1,000 sccm is preferred. The time can
range from 20 seconds up to ten minutes, however 5 minutes is generally
acceptable.
The reaction pressure must be subatmospheric and generally will vary from
about 500 millitorr to about 1 torr. If one desired to decrease the time,
the flow rate and temperature could be increased. Likewise, with reduced
temperature increased time is preferred. Likewise, when reducing the
temperature, the RF frequency can also be reduced. Plasma power can be
increased or decreased, likewise, to alter the time or reaction rate.
Although not limited to any particular apparatus, one preferred apparatus
for use in the present invention is a chemical vapor deposition reactor 20
shown in the FIGURE. Alternate embodiments of the reaction are disclosed
in U.S. Patent Application entitled "Method and Apparatus for the
Efficient Use of Reactant Gases and Plasmas for Depositing CVD and PECVD
Films," filed on even date herewith and listing Robert F. Foster, Joseph
T. Hillman, Rikhit Arora as inventors. The disclosure of this application
is incorporated herein by reference.
Reactor 20, and specifically reaction space 24 within housing 22, may be
selectively evacuated to various different internal pressures--for
example, from 0.5 to 100 Torr. The susceptor 26 is coupled to a variable
speed motor (not shown) by shaft 30 such that the susceptor 26 and
substrate 28 may be rotated at various speeds such as between 0 and 2,000
rpm. Susceptor 26 includes a resistance heating element (not shown)
coupled to the susceptor 26 to heat substrate 28 to between 200.degree.
and 800.degree. C.
Extending downwardly from the top wall 32 of housing 22 is a cylinder
assembly 34 which is attached to a gas-dispersing showerhead 36.
Showerhead 36 is coupled to an RF energy source 38 by an appropriate RF
feedline assembly 40 which extends through cover 46 which may, if
necessary, include a heat pipe to dissipate unwanted heat. A sealing
structure 49 seals the opening around feedline assembly 40. Plasma and
reactant gases are introduced into flow passage 44 by concentric rings or
halos 50, 52. The concentric rings 50, 52 include a number of holes which
evenly dispense the gases around the flow passage 44. Ring 50 is connected
to a gas supply through line 56, while ring 52 is connected to a supply by
line 58.
An insulator ring 62 separates cylinder 34 and showerhead 36 for reasons
discussed hereinbelow. Cylinder 34 is electrically grounded by ground line
61.
The insulator ring 62 preferably has an outer diameter approximately the
same as the outer diameter of showerhead 36 and a width dimension which
ensures complete separation of cylinder 34 and showerhead 36 along the
entire attachment interface between the cylinder and showerhead. The
insulator ring is preferably made of quartz material approximately 0.75
inches thick.
Showerhead/electrode 36 contains a plurality of dispersion holes 64 which
disperse the flow of gas over substrate 28.
The showerhead 36 includes a stem 68. Stem 68 is formed integrally with the
showerhead 36 and forms part of the RF line assembly 40 which connects to
showerhead 36. The showerhead 36, including stem 68, is formed of an
electrically conductive material, preferably Nickel-200. As may be
appreciated, other conductive materials may also be appropriate.
The RF power source, through RF feedline assembly 40, biases the showerhead
36 so that the showerhead functions as an RF electrode, The grounded
susceptor 26 forms another parallel electrode. An RF field is created,
preferably between showerhead 36 and susceptor 26. Hereinafter in the
application, showerhead 36 will be referred to as showerhead/electrode 36
when referring to a biased showerhead 36 in accordance with the principles
of the present invention. The RF field created by the biased
showerhead/electrode 36 excites the plasma gases which are dispensed
through holes 64 so that a plasma is created below showerhead/electrode
36.
The showerhead employed is about 0.25 inches thick, having a diameter of
about 17.3 cm and 600 holes. The number of holes is not critical and could
easily be varied from 100 holes to 1,000 or more holes. The holes are
preferably less than 1.5 mm in diameter and are more preferably about 0.75
min. This prevents the plasma from being generated in the hole, thereby
reducing efficiency.
The gas flow from injector rings 50 and 52 is allowed to develop within the
length of the cylinder 34 as it travels to the showerhead 36. It is
desirable for the velocity profile of the incoming plasma gases passing
through showerhead 36 to be fully developed before they reach the rotating
surface of the substrate 28. Due to the proximity of the showerhead to the
surface, that profile must develop in the cylinder 34.
Utilizing cylinder 34 shown in the FIGURE, the showerhead-to-susceptor
spacing may be reduced to approximately 30 to 20 mm or less because the
velocity profile develops in cylinder 34, Therefore, the length of
cylinder 34 from the injector rings 50 and 52 to showerhead 36 should be
40 to 100 mm. As the gases pass through the showerhead 36, the pressure
drop across the showerhead 36 flattens out the velocity profile of the
gases. As the gases approach showerhead/electrode 36 and pass
therethrough, they are excited into a plasma which contacts surface 29.
The showerhead 36 can be from about 10 cm to about 10 millimeters from the
susceptor, with 20 mm preferred. It is preferred to have the showerhead as
close as possible to the substrate while still permitting the substrate or
wafer to be removed.
A pumping effect is created by the rotating susceptor 26. The plasma
radicals and ions (nitrogen and hydrogen) are drawn to the upper surface
29 of substrate 28. Generally, the rotation rate can vary from 0 rpm to
1500 rpm. About 100 rpm is preferred. Further, matched flow does not
appear to be critical but can be employed.
With a spacing of about 25 mm between the showerhead and the substrate 28,
the created plasma is much closer to the titanium surface 29. With the
showerhead 36 acting as an RF electrode, a more uniform plasma is
generated, therefore enhancing the uniformity of radical and ion density
at the substrate 28 and thereby improving nitridization reaction rate.
When employing this apparatus, the electrode is biased--generally at a
frequency from about 13.56 MHz (a frequency which is authorized by the
Federal Communication Commission)--down to about 400 KHz. The power of the
electrode is generally set at about 250 watts. The substrate with the
titanium coating is placed on the susceptor and heated to about
650.degree. C. or less.
The nitridization gas, preferably ammonia, is introduced through injectors
50 and 52 and flows through the cylinder 34 and through showerhead 36,
which creates the plasma from the gas. The flow rate of the gas into
cylinder 34 is generally about 1,000 sccm and the pressure within the
reaction chamber itself is maintained at about 1 to 3 torr (3 is
preferred). The heated susceptor is rotated at a rotation rate of about
100 rpm which, in effect, pumps gas laterally away from the titanium
surface 29 as the plasma is forced downwardly toward the titanium surface.
This reaction continues for about five minutes. Unreacted ammonia, along
with hydrogen, will (as shown by arrows 65) be pulled around baffles 27
and from the reaction chamber 14 through vent 53.
The titanium film 29 will take on a gold luster, indicating the formation
of titanium nitride. The titanium nitride film can then be further treated
in the reaction chamber or can be removed to a separate chamber for
further treatment.
If treated in this reaction chamber, one preferred further treatment would
be the plasma-enhanced chemical vapor deposition of titanium nitride,
using as reactants titanium tetrachloride, and nitrogen or ammonia.
The present invention will be further appreciated in light of the following
examples:
EXAMPLE 1
A plasma vapor deposited titanium film was annealed under the following
conditions:
______________________________________
Substrate temperature
550.degree. C. (650.degree. C. susceptor)
Plasma power 250 watts
Plasma frequency 15.5 MHz
Ambient gas ammonia flow rate
1000 sccm
Pressure 1 torr
Time 5 minutes
Rotation Rate 100 rpm
______________________________________
The annealed titanium film was removed from the reaction chamber and
inspected. The silver-grey color of the titanium had taken on a dull
yellow-gold that indicated conversion to titanium nitride.
The film was loaded back into the reaction chamber for chemical vapor
deposition of titanium nitride using a two-step deposition at 480.degree.
C.
In step 1, the process was run in TiCl.sub.4 depletion, i.e., TiCl.sub.4
flow rate of 20 sccm, 500 sccm NH.sub.3, 5 liters per minute N.sub.2.
Approximately 100 angstroms of TiN was deposited with an expected chlorine
content of 3% and less than 100% conformality. In step 2, the TiCl.sub.4
flow is turned up into the saturation regime, 80 sccm TiCl.sub.4 with
NH.sub.3 and N.sub.2 rates constant. In this step, 400 angstroms of TiN is
deposited with 4.5% chlorine and 100% conformality.
In this case, the result was an excellent quality TiN deposition over the
nitrided titanium film.
EXAMPLE 2
A plasma vapor deposited titanium film was annealed at about 450.degree. C.
(520.degree. C. susceptor). The other anneal conditions were as follows:
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Substrate temperature
450.degree. C. (520.degree. C. susceptor)
Plasma power 250 watts
Plasma frequency 440 KHz
Ambient gas ammonia flow rate
1000 sccm
Pressure 1 torr
Time 5 minutes
Rotation Rate 100 rpm
______________________________________
After annealing, the sample was removed for inspection, The color had
changed to a deep gold. This indicated that a higher quality titanium
nitride was formed despite the reduction in temperature.
After inspection, the sample was placed back into the CVD chamber for TiN
deposition. The deposition process was exactly the same as described in
Example 1. The result was an excellent TiN deposition over the annealed
titanium, No cracking, peeling or pitting were observed.
Thus, by employing the method of the present invention the temperature of
the anneal can actually be reduced by decreasing the frequency, This, in
turn, improves the nitridization reaction, thereby preventing degradation
of the underlying substrate.
This has been a description of the present invention, along with the
preferred embodiment of practicing the present invention. However, the
invention itself should only be defined by the appended claims wherein
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